The ability of a light source to render deep red colors is measured by the metric R9. Unfiltered incandescent light sources by definition render those extremely well, typically greater than 97. Replacements for incandescent light sources have tended to struggle with faithful rendition of red colors. For example high pressure sodium lamps and older fluorescent lighting tubes often had negative values for R9, and reduced almost any red color to a rather dull orange appearance. Early light emitting diodes (LEDs) were notorious for poor rendering of red colors. The situation was so significant that many programs related to LED lighting only require R9>0. This contrasts with the requirements for the general color rendering index (CRI) which are usually CRI>80.
Generally speaking, general lighting can be made more pleasing by the increase of red-green contrast, for example by the removal of yellow light, which can wash out the appearance of many objects. This phenomenon has been known in the art for many years, dating back at least to U.S. Pat. No. 4,441,046 “Incandescent lamps with neodymium oxide vitreous coatings”, in which a neodymium oxide coating filters out some of the green and lot of the yellow. Based on this work, in 1995 GE released the Enrich® line of incandescent light bulbs, and in 2001 renamed the line Reveal®. The incandescent Reveal® product line still exists today, along with an updated Reveal® LED product, still utilizing a neodymium based filter.
FIG. 1 shows normalized emission spectra of a Reveal® incandescent bulb (solid line) and of a Reveal® A19 LED bulb (dashed line), showing the effect of the neodymium oxide filters in these products. One of the largest drawbacks of this methodology is that it first creates photons and then removes a significant portion of the photons which have been created. This can be seen in the rated outputs of a 60 W Reveal® incandescent, 520 lumens, and Reveal® LED A19, 570 lumens, compared with the benchmark 800 lumens for a 60 W equivalent A19.
Generally, it has been an industry goal to produce white light emitting phosphor-converted LEDs that have emission spectra that are relatively flat, sloped, and continuous in the region between about 500 nanometers (nm) and about 600 nm. This general shape roughly mirrors the reference illuminant, e.g. the emission spectra of black body radiators such as a standard incandescent. As shown in FIG. 1, the neodymium oxide filter used in the Reveal® products introduces a dip in the emission spectra in the yellow region. Such a dip may be characterized by the residual intensity at its minimum when compared to the maximum intensity of the emission spectrum between 400 nm and 700 nm, for example about 25% for the incandescent version and about 33% for the LED A19 version.
Red-green contrast does not have a clear metric in the CRI/Ra system, however it can be captured to some extent by the gamut index metric (Rg) of the IES TM-30-15 method. Applicant measured an Rg of 109 for the Reveal® incandescent versus 101 for a non-filtered incandescent. The Reveal® LED bulb similarly measures at a high Rg value of 104. Surprisingly, despite the good gamut indices, these bulbs measure relatively poorly on the R9 deep red metric. A drawback of this method is that the neodymium filter used subtracts a significant amount of the light generated. Photons in the wavelength region impacted by these neodymium filters are especially particularly bright, typically ranging from 512 to 625 lumens per optical Watt compared with the maximum of 683 lumens per optical Watt. The Reveal® LED bulb is rated to deliver 570 lumens using 10.5 W, while a similar Correlated Color Temperature (CCT) and CRI LED bulb from the Relax® line delivers 800 lumens using those same 10.5 W.
Typically, white light emitting phosphor-converted LEDs comprise a two or sometimes three phosphor blend, with a combination of a broad green or yellow phosphor having a full width at half maximum (FWHM) of about 60-100 nm and a peak wavelength of about 500-570 nm and a broad red phosphor having a FWHM of about 70-100 nm and a peak wavelength of about 615-670 nm, or more usually about 625-650 nm.
A red phosphor with peak emission at 625-630 nm provides higher efficacy, due to the better overlap of the red phosphor emission and the photopic response curve of the typical human eye, but this choice of red phosphor emission is generally to the detriment of R9. Conversely, a red phosphor with a peak emission closer to 650 nm provides better red rendering, however, but at a cost of efficacy because the longer wavelength red emission contributes little to the overall brightness of the LED. There is generally an inverse relation between the deep red rendering of a light source as measured by R9 and the spectral efficiency or luminous efficacy of radiation (LER) of the spectrum.